1. Field of the Invention
The present invention relates to a voltage converter, and more specifically, to a buck converter.
2. Description of the Prior Art
Normally, there are only a few main power supplies for electronic devices. These include, for example, 12V and 9V; however, various integrated circuits providing a variety of different functions are implemented in systems of electronic devices that do not utilize said power supplies but rather different power sources, such as 5V and 3V. Therefore, a set of buck converters is needed to convert the main power supply into a variety of other required voltages.
Referring to FIG. 1, FIG. 1 illustrates a circuit diagram of a prior art buck converter 100. A power source Vin is provided to an input terminal of a transistor 101. Transistor 101 includes a control terminal and an output terminal, wherein the output terminal is electrically connected to a first terminal of diode 102; and a second terminal of diode 102 is connected to ground. A first terminal of an inductor 103 is electrically connected to the output terminal of transistor 101 while the second terminal and a power output terminal are electrically connected to the first terminal of capacitor 104. In addition, the second terminal of capacitor 104 is electrically connected to ground. Referring to FIG. 2, FIG. 2 illustrates a timing diagram of a control signal VG1 received from the control terminal of transistor 101 and an output voltage Vo at the power output terminal. As shown in FIG. 2, when control signal VG1 corresponds to a high voltage level, output voltage Vo increases gradually; when control signal VG1 corresponds to a low voltage level, output voltage Vo decreases gradually. In the prior art, the relationship between the average output voltage Vo and power source Vin is shown as below:Vo/Vin=D  Equation (1)In Equation (1), D is equal to Ton/T, which is the duty cycle of the control signal VG1. Therefore, a circuit designer is capable of adjusting the duty cycle of the control signal VG1 to obtain the required output voltage Vo.
It should be noted that output voltage Vo includes ripples, and the magnitude of the ripples will degrade the output power quality of the buck converter. In order to decrease the magnitude of the ripples, the prior art increases the frequency of the control signal VG1. Although this does decrease the ripples, a consequence is that a higher frequency will increase the on/off switching frequency of transistor 101. As a result, the switching loss increases. Furthermore, unwanted heat is generated due to the high on/off switching frequency. This unwanted heat will decrease the life of the transistor 101.
Referring to FIG. 3. FIG. 3 illustrates a circuit diagram of another prior art buck converter 300. Prior art buck converter 100 differs from prior art buck converter 300 in that the transistor 301 and the transistor 302 are jointly connected to a control signal source 307, and control signal source 307 generates a control signal to turn on or turn off both transistor 301 and transistor 302 simultaneously. Accordingly, the power stress imposed on transistor 301 and transistor 302 will decrease. However, the switching loss caused by the high on/off frequency of transistor 301 and transistor 302 still exists.